Regulation of body osmolality helps to maintain proper fluid balance, cardiovascular function and life. Neurons located in the organum vasculosum of the lamina terminalis (OVLT) and ventral median preoptic nuclei (vMnPO) in the hypothalamus play a critical role in fluid and electrolyte balance. It has been hypothesized that multiple neurons, each intrinsically osmosensitive form a dynamic pathway from the OVLT/vMnPO to the supraoptic nuclei (SON) that regulates endocrine responses to altered states of osmolality. While it is clear that magnocellular neurons in the SON are intrinsically sensitive to osmotic stimuli, it is unclear if glutamatergic projection neurons from the OLVT/vMnPO are also intrinsically osmosensitive. Further, it is unclear if osmosensitive OVLT/vMnPO neurons express key genes encoding channel proteins thought to be involved in osmotic transduction mechanisms. Patch clamp recordings of dissociated and cultured OVLT/vMnPO neurons backlabeled from the SON will provide information about the neurophysiological properties of osmosensation in this neuronal population. Single cell molecular biology will then allow us to determine if neurons projecting to the SON are intrinsically osmosensitive and glutamatergic. Finally, osmosensitive neurons will be probed for expression of genes encoding channel proteins thought to be involved in osmosensation. These experiments will help us better define the functional organization, gene expression and neurophysiological mechanisms underlying osmosensation in OVLT/vMnPO neurons.